U.S. patent application number 11/792033 was filed with the patent office on 2007-12-27 for switching regulator control circuit, current drive circuit, light emitting apparatus, and information terminal apparatus.
This patent application is currently assigned to ROHM CO., LTD.. Invention is credited to Tomoyuki Ito, Isao Yamamoto.
Application Number | 20070296353 11/792033 |
Document ID | / |
Family ID | 36564876 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070296353 |
Kind Code |
A1 |
Ito; Tomoyuki ; et
al. |
December 27, 2007 |
Switching Regulator Control Circuit, Current Drive Circuit, Light
Emitting Apparatus, and Information Terminal Apparatus
Abstract
A control circuit may include a first feedback input terminal
which receives the cathode terminal voltage of light-emitting
elements from a current driving circuit as a feedback signal. Such
an arrangement controls the ON/OFF state of a switching element
such that the cathode terminal voltage approaches a predetermined
voltage. A second feedback input terminal may be included to
receive the anode terminal voltage of the light-emitting elements
as a feedback signal. Such an arrangement controls the ON/OFF state
of the switching element such that the anode terminal voltage does
not exceed a predetermined threshold voltage. A feedback output
terminal may be included of the current driving circuit which
allows the cathode terminal voltage of the light-emitting elements
to be input to a control circuit for the switching regulator as a
feedback signal. The control circuit and the current driving
circuit may be integrally provided in the form of separate
semiconductor chips.
Inventors: |
Ito; Tomoyuki; (Kyoto,
JP) ; Yamamoto; Isao; (Kyoto, JP) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Assignee: |
ROHM CO., LTD.
21, SAIIN MIZOSAKI-CHO
UKYRO-KU, KYOTO
JP
615-8585
|
Family ID: |
36564876 |
Appl. No.: |
11/792033 |
Filed: |
October 26, 2005 |
PCT Filed: |
October 26, 2005 |
PCT NO: |
PCT/JP05/19672 |
371 Date: |
May 30, 2007 |
Current U.S.
Class: |
315/307 ;
361/837 |
Current CPC
Class: |
H05B 45/46 20200101;
H05B 45/347 20200101; H02M 3/156 20130101; H05B 45/38 20200101;
G09G 3/3413 20130101 |
Class at
Publication: |
315/307 ;
361/681 |
International
Class: |
G05F 1/00 20060101
G05F001/00; G06F 1/16 20060101 G06F001/16; H05B 39/04 20060101
H05B039/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2004 |
JP |
2004-345979 |
Claims
1. A control circuit for a switching regulator that drives a
light-emitting element, comprising a first feedback input terminal
which receives the cathode terminal voltage as a feedback signal
from a current driving circuit connected to the cathode terminal of
said light-emitting element, wherein the ON/OFF state of a
switching element of said switching regulator is controlled such
that the cathode terminal voltage approaches a predetermined
voltage.
2. A control circuit according to claim 1, further comprising a
second feedback terminal which receives the anode terminal voltage
of said light-emitting element as a feedback signal, wherein the
ON/OFF state of said switching element is controlled such that the
anode terminal voltage does not exceed a predetermined threshold
voltage.
3. A current driving circuit which is connected to the cathode
terminal of a light-emitting element driven by a switching
regulator, and which controls the current that flows through said
light-emitting element, comprising: a constant-current circuit
connected to the cathode terminal of said light-emitting element;
and a feedback output terminal which inputs the cathode terminal
voltage of said light-emitting element to a control circuit of said
switching regulator as a feedback signal.
4. A current driving circuit according to claim 3, which includes a
plurality of said light-emitting elements, further comprising a
minimum value circuit which outputs a lowest voltage from among the
voltages output from the cathode terminals of said light-emitting
elements, wherein the output of said minimum value circuit is input
via said feedback output terminal as a feedback signal.
5. A light-emitting device comprising: a light-emitting element; a
switching regulator which is connected to the anode terminal of
said light-emitting element, and which generates the driving
voltage for driving said light-emitting element; a current driving
circuit which is connected to the cathode terminal of said
light-emitting element, and which controls the current that flows
through said light-emitting element; and a control circuit which
controls the ON/OFF state of a switching element of said switching
regulator such that the voltage of said cathode terminal received
from said current driving circuit as a feedback signal approaches a
predetermined voltage, wherein said current driving circuit and
said control circuit are integrally provided in the form of
separate semiconductor chips.
6. A light-emitting device according to claim 5, which is included
within an information terminal device having a structure in which a
first housing and a second housing are foldably connected with each
other via a connection portion, wherein said light-emitting element
and said current driving circuit are mounted within said first
housing that includes a liquid crystal panel that operates using
said light-emitting element as a backlight, and wherein said
switching regulator and a control circuit for said switching
regulator are mounted within said second housing, and wherein a
first wiring line, which connects said switching regulator and the
anode terminal of said light-emitting element and which supplies
the driving voltage, and a second wiring line, which connects said
current driving circuit and said control circuit and which allows
the voltage of the cathode terminal of said light-emitting element
to be input as a feedback signal, are provided between said first
housing and said second housing via said connection portion.
7. An information terminal device having a structure in which a
first housing and a second housing are foldably connected with each
other via a connection portion, wherein said first housing
comprises: a liquid crystal panel; and a current driving circuit
which is connected to the cathode terminal of said light-emitting
element, and which controls the current that flows through said
light-emitting element, and wherein said second housing comprises:
a battery; a switching regulator which generates a driving voltage
for driving said light-emitting element using the voltage of said
battery; and a control circuit which controls the ON/OFF state of a
switching element of said switching regulator such that the voltage
of the cathode terminal input from said current driving circuit as
a feedback signal approaches a predetermined voltage.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a control circuit for a
switching regulator for driving light-emitting elements, a current
driving circuit, and a light-emitting device including the control
circuit and the current driving circuit.
[0003] 2. Description of the Related Art
[0004] In recent years, information terminals such as cellular
phones, PDAs (Personal Digital Assistants), etc., have come to
include light emitting diodes (which will be referred to as "LEDs"
hereafter) used as a backlight for a liquid crystal panel. In many
cases, such information terminals employ a lithium-ion battery.
However, in the normal state, such a lithium-ion battery provides
output voltage of around 3.5 V. Even immediately after being fully
charged, the lithium-ion battery provides only around 4.2 V. On the
other hand, a voltage of 4 V or more is required to drive LEDs.
Accordingly, there is a need to boost the battery voltage using a
power supply device such as a switching regulator or the like as
necessary in order to supply the necessary voltage to the LEDs.
[0005] In order to drive the LEDs by way of a load circuit using
such a switching regulator, a current driving circuit is connected
to the cathode terminals of LEDs, and feedback control is performed
so as to maintain at a constant value the current that flows
through the LEDs. Examples of such feedback control methods include
a method which employs a detection resistor. With such an
arrangement, while the LEDs are being driven, feedback control is
performed so as to maintain at a constant value the voltage drop
across the detection resistor (see Patent document 1). Also,
examples of such feedback control methods include a method which
employs a constant-current circuit that allows a predetermined
current to flow through the LEDs, instead of the detection
resistor. Such an arrangement also provides a constant driving
current that flows through the LEDs (see Patent document 2).
[0006] As described above, such an arrangement for driving LEDs
requires a switching regulator, a control circuit for controlling
the switching operation of the switching regulator, and a current
driving circuit connected to the cathode terminals of the LEDs.
[Patent Document 1]
[0007] Japanese Patent Application Laid-open No. 2003-152224
[Patent Document 2] [0008] Japanese Patent Application Laid-open
No. 2004-22929
[0009] In such circumstances, the present inventor has come to
recognize the following problems.
[0010] With regard to the information terminals such as cellular
phones, in recent years, the mainstream models have come to be
folding models, which have a structure in which a first housing
including a liquid crystal panel and a second housing including an
operation unit are connected to each other. Let us consider an
arrangement in which, of the peripheral circuits for driving the
LEDs mounted on such a folding information terminal, a control
circuit for a switching regulator and a current driving circuit are
integrally provided in the form of an LED driving circuit. With
such an arrangement, the LED driving circuit and the peripheral
components thereof are mounted on the first housing side on which a
liquid crystal panel is mounted.
[0011] On the other hand, the switching regulator requires, in the
form of peripheral components: an inductor for performing energy
conversion by receiving current supplied from a switching element;
and a smoothing capacitor. Specifically, such components are
mounted in the form of chip components. However, such an
arrangement leads to a problem that the thickness of the first
housing is dependent upon the heights of these peripheral
components. On the other hand, from the commercial perspective,
there is a great demand for smaller and slimmer information
terminals. Accordingly, for such information terminals, it can be
understood that this problem is a critical problem to be
solved.
SUMMARY OF THE INVENTION
[0012] The present invention has been made in view of such a
problem. Accordingly, it is a general purpose of the present
invention to provide a control circuit for controlling a switching
regulator, a current driving circuit, and a light-emitting device,
which are mounted on a folding information terminal, and which
provide a slimmer information terminal.
[0013] An embodiment of the present invention relates to a control
circuit for a switching regulator which drives a light-emitting
element. The control circuit comprises a first feedback input
terminal which receives the cathode terminal voltage as a feedback
signal from a current driving circuit connected to the cathode
terminal of the light-emitting element. With such an arrangement,
the ON/OFF state of a switching element of the switching regulator
is controlled such that the cathode terminal voltage approaches a
predetermined voltage.
[0014] The term "light-emitting element" as used here represents an
element having a function of providing variable brightness which
can be controlled by adjusting the current or voltage, examples of
which include light-emitting diodes, organic EL elements, laser
diodes, etc. With such an arrangement, the control circuit for the
switching regulator includes the first feedback input terminal for
receiving the cathode terminal voltage of the light-emitting
element as a feedback signal. Furthermore, the control circuit is
provided in the form of a separate integrated circuit that differs
from the current driving circuit for driving the light-emitting
element. Such an arrangement improves the degree of freedom in the
layout of the light-emitting element, the switching regulator, the
current driving circuit, and the peripheral components.
[0015] Also, the control circuit may further comprise a second
feedback terminal which receives the anode terminal voltage of the
light-emitting element as a feedback signal. Also, the ON/OFF state
of the switching element may be controlled such that the anode
terminal voltage does not exceed a predetermined threshold
voltage.
[0016] With such an arrangement, the second feedback input terminal
is provided for monitoring the anode terminal voltage. Let us
consider a case in which the wiring line that connects the first
feedback input terminal and the current driving circuit has broken,
which leads to a situation in which the cathode terminal voltage of
the light-emitting elements cannot be monitored. Even in such a
case, such an arrangement prevents the driving voltage output from
the switching regulator from increasing beyond a predetermined
threshold voltage.
[0017] Another embodiment of the present invention relates to a
current driving circuit which is connected to the cathode terminal
of a light-emitting element driven by a switching regulator, and
which controls the current that flows through the light-emitting
element. The current driving circuit comprises: a constant-current
circuit connected to the cathode terminal of the light-emitting
element; and a feedback output terminal which inputs the cathode
terminal voltage of the light-emitting element to a control circuit
of the switching regulator as a feedback signal.
[0018] With such an arrangement, the current driving circuit for
the light-emitting element includes the feedback output terminal
for outputting the cathode terminal voltage of the light-emitting
element. Furthermore, the current driving circuit is provided in
the form of a separate integrated circuit that differs from the
control circuit for the switching regulator. Such an arrangement
improves the degree of freedom in the layout of the light-emitting
element, the switching regulator, the current driving circuit, and
the peripheral components.
[0019] Let us consider an arrangement in which multiple
light-emitting elements are employed. With such an arrangement, the
current driving circuit may further comprises a minimum value
circuit which outputs a lowest voltage from among the voltages
output from the cathode terminals of the light-emitting elements.
Also, the output of the minimum value circuit may be input via the
feedback output terminal as a feedback signal.
[0020] With such an arrangement, the lowest voltage among the
cathode terminal voltages of the light-emitting elements is output
as a feedback voltage input from the current driving circuit to the
control circuit for the switching regulator. Such an arrangement
offers stable operation of all the constant-current circuits.
[0021] Yet another embodiment of the present invention relates to a
light-emitting device. The light-emitting device comprises: a
light-emitting element; a switching regulator which is connected to
the anode terminal of the light-emitting element, and which
generates the driving voltage for driving the light-emitting
element; a current driving circuit which is connected to the
cathode terminal of the light-emitting element, and which controls
the current that flows through the light-emitting element; and a
control circuit which controls the ON/OFF state of a switching
element of the switching regulator such that the voltage of the
cathode terminal received from the current driving circuit as a
feedback signal approaches a predetermined voltage. With such an
arrangement, the current driving circuit and the control circuit
are integrally provided in the form of separate semiconductor
chips.
[0022] Such an arrangement improves the degree of freedom in the
layout of the light-emitting element, the switching regulator, the
current driving circuit, and the peripheral components.
[0023] Also, the light-emitting device may be mounted within an
information terminal device having a structure in which a first
housing and a second housing are foldably connected with each other
via a connection portion. Also, the light-emitting element and the
current driving circuit may be mounted within the first housing
that includes a liquid crystal panel that operates using the
light-emitting element as a backlight. Also, the switching
regulator and a control circuit for the switching regulator may be
mounted within the second housing. Also, a first wiring line, which
connects the switching regulator and the anode terminal of the
light-emitting element and which supplies the driving voltage, and
a second wiring line, which connects the current driving circuit
and the control circuit and which allows the voltage of the cathode
terminal of the light-emitting element to be input as a feedback
signal, are provided between the first housing and the second
housing via the connection portion.
[0024] Yet another embodiment of the present invention relates to
an information terminal device having a structure in which a first
housing and a second housing are foldably connected with each other
via a connection portion. With such an arrangement, the first
housing comprises: a liquid crystal panel; and a current driving
circuit which is connected to the cathode terminal of the
light-emitting element, and which controls the current that flows
through the light-emitting element. On the other hand, the second
housing comprises: a battery; a switching regulator which generates
a driving voltage for driving the light-emitting element using the
voltage of the battery; and a control circuit which controls the
ON/OFF state of a switching element of the switching regulator such
that the voltage of the cathode terminal input from the current
driving circuit as a feedback signal approaches a predetermined
voltage.
[0025] With such an arrangement, the light-emitting elements and
the current driving circuit are mounted on the first housing side,
i.e., the liquid crystal panel side. On the other hand, the other
circuit components are mounted on the second housing side. Such an
arrangement provides a slimmer first housing.
[0026] Note that any combination of the aforementioned components
or any manifestation of the present invention realized by
replacement of a method, a device, a system, and so forth, is
effective as an embodiment of the present invention.
[0027] It is to be noted that any arbitrary combination or
rearrangement of the above-described structural components and so
forth is effective as and encompassed by the present
embodiments.
[0028] Moreover, this summary of the invention does not necessarily
describe all necessary features so that the invention may also be a
sub-combination of these described features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Embodiments will now be described, by way of example only,
with reference to the accompanying drawings which are meant to be
exemplary, not limiting, and wherein like elements are numbered
alike in several Figures, in which:
[0030] FIG. 1 is an external view of a cellular phone terminal on
which a light-emitting device is mounted according to an embodiment
of the present invention.
[0031] FIG. 2 is a circuit diagram which shows a configuration of
the light-emitting device according to the present embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The invention will now be described based on preferred
embodiments which do not intend to limit the scope of the present
invention but exemplify the invention. All of the features and the
combinations thereof described in the embodiment are not
necessarily essential to the invention.
[0033] FIG. 1 is an external view of a cellular phone terminal 1000
on which a light-emitting device is mounted according to an
embodiment of the present invention. The cellular phone terminal
1000 includes a first housing 1100 and a second housing 1200, which
are connected to each other using a connection portion 1300 having
a hinged structure, thereby providing a structure which allows a
user to fold the cellular phone terminal.
[0034] A liquid crystal panel 1400 is disposed on the first housing
1100 side. On the other hand, operation buttons 1500 are disposed
on the second housing 1200 side. A battery that serves as a power
supply for the cellular phone terminal 1000 is provided to the rear
face side of the second housing 1200.
[0035] The light-emitting device according to the present
embodiment includes light-emitting diodes used as a backlight for
the liquid crystal panel 1400, and a peripheral circuit for the
light-emitting diodes.
[0036] FIG. 2 is a circuit diagram which shows a configuration of a
light-emitting device 400 according to the present embodiment.
[0037] The light-emitting device 400 includes light-emitting diodes
300R, 300G, and 300B, that correspond to the three primary colors,
i.e., red, green, and blue (RGB). With such an arrangement, the
light-emitting diodes that correspond to the respective three
primary colors are turned on at the same time so as to provide
white light, thereby offering a function as a backlight for the
liquid crystal panel 1400.
[0038] The light-emitting device 400 includes: a current driving
circuit 200 for controlling currents that flow through the
light-emitting diodes 300R, 300G, and 300B; a switching regulator
130 for supplying driving currents to the light-emitting diodes
300R, 300G, and 300B; and a control circuit 100 for controlling the
ON/OFF state of the switching element of the switching regulator
130.
[0039] The switching regulator 130 is a step-up DC/DC converter
having a function of boosting the battery voltage Vbat output from
a battery 500, thereby providing the voltage thus boosted as the
output voltage Vout. The switching regulator 130 is connected to
the anode terminals of the light-emitting diodes 300R, 300G, and
300B, and supplies the output voltage Vout to the light-emitting
diodes 300R, 300G, and 300B.
[0040] The switching regulator 130 includes an inductor L1, a
rectifier diode D1, an output capacitor C1, and a switching element
SW1.
[0041] The inductor L1 and the switching element SW1 are connected
with each other in series between the battery 500 and the ground
potential. The switching element SW1 comprises a MOS transistor,
which switches ON/OFF according to a pulse-width modulated
switching signal that is applied to the gate terminal thereof.
[0042] Furthermore, the anode terminal of the rectifier diode D1 is
connected to the connection between the inductor L1 and the
switching element SW1. The output capacitor C1 is connected between
the cathode terminal of the rectifier diode D1 and the ground
potential.
[0043] With regard to the switching regulator 130, when the
switching element SW1 is ON, a current flows from the battery 500
to the switching element SW1 via the inductor L1. On the other
hand, when the switching element SW1 is OFF, the inductor L1
provides an inductive effect to maintain the current flow as it is
during the ON state of the switching element SW1. Such a current
flows toward the output capacitor C1. Specifically, the current is
supplied to the output capacitor C1 via the rectifier diode D1,
thereby charging the output capacitor C1.
[0044] With such an arrangement, the switching element SW1 is
repeatedly switched ON/OFF, thereby providing energy conversion
between the inductor L1 and the output capacitor C1. Thus, the
battery voltage Vbat, which is an input voltage, is boosted, and
the battery voltage thus boosted is smoothed by the output
capacitor C1, thereby outputting the voltage thus smoothed as the
output voltage Vout.
[0045] The current driving circuit 200 is connected to the cathode
terminals of the light-emitting diodes 300R, 300G, and 300B. The
current driving circuit 200 includes the constant-current circuits
210R, 210G, and 210B, and a minimum value circuit 220.
[0046] The constant-current circuits 210R, 210G, and 210B are each
connected to the cathode terminals of the three light-emitting
diodes 300R, 300G, and 300B, which correspond to red (R), green
(G), and blue (B), respectively. The constant-current circuits
210R, 210G, and 210B, are connected to the light-emitting diodes
300R, 300G, and 300B, respectively, each of which controls the
current that flows through the corresponding light-emitting
diode.
[0047] The minimum value circuit 220 receives the voltages of the
cathode terminals of the three light-emitting diodes 300R, 300G,
and 300B, as input voltages. The minimum value circuit 220 outputs
the lowest voltage from among these input voltages. The first
feedback voltage Vfb1, which is the output of the minimum value
circuit 220, is output from a feedback output terminal 208, and is
input to the control circuit 100 as a feedback signal.
[0048] The control circuit 100 includes a pulse-width modulation
circuit (which will be referred to as a "PWM circuit" hereafter)
110, and a driver circuit 120. With such a configuration, the
control circuit 100 creates a switching signal for controlling the
ON/OFF state of the switching element SW1 of the switching
regulator 130, and outputs the switching signal thus created from a
switching signal output terminal 108.
[0049] A first feedback input terminal 104 receives the first
feedback voltage Vfb1 output as a feedback signal from the feedback
output terminal 208 of the current driving circuit 200. A second
feedback input terminal 106 receives, as the second feedback
voltage Vfb2, the output voltage Vout output from the switching
regulator 130.
[0050] The first feedback voltage Vfb1 is input to the PWM circuit
110. The PWM circuit 110 creates a pulse-width modulated PWM signal
Vpwm with a constant frequency and with a variable ON time adjusted
according to the first feedback voltage Vfb1. The PWM circuit 110
outputs the PWM signal Vpwm thus created to the driver circuit 120.
The PWM circuit 110 creates the PWM signal Vpwm such that the first
feedback voltage Vfb1 approaches a predetermined reference voltage
Vref. Here, detailed description will be omitted regarding the PWM
circuit 110. In brief, the PWM circuit 110 comprises: a computation
amplifier which amplifies the difference between the first feedback
voltage Vfb1 and the reference voltage Vref; and a voltage
comparator which makes a comparison between the output Verr output
from the computation amplifier and the triangular wave Vosc output
from an oscillator. With such an arrangement, in a case that the
Verr is greater than the Vosc, the voltage comparator outputs a
high-level signal. On the other hand, in a case that the Verr is
smaller than the Vosc, the voltage comparator outputs a low-level
signal. Such an arrangement provides a pulse-width modulated signal
with a variable high-level period, i.e., with a variable ON-time
adjusted according to the voltage Verr.
[0051] Each of the constant-current circuits 210R, 210G, and 210B
includes a transistor for current control between a corresponding
terminal 230R, 230G, or 230B and a corresponding ground terminal
202R, 202G, or 202B. Saturation of the transistor occurs in a case
that the voltage between the drain and the source of the transistor
or the voltage between the collector and the emitter becomes equal
to or smaller than a predetermined voltage. This leads to a
situation in which stable electric current cannot be generated.
With such an arrangement, the voltage to be applied to the cathode
terminals of the light-emitting diodes 300R, 300G, and 300B, i.e.,
the reference voltage Vref (which serves as the target value of the
first feedback voltage Vfb1), is preferably set to the smallest
possible value in the range of voltages that permit stable
operation of the constant-current circuits 210R, 210G, and 210B
without the transistors becoming saturated.
[0052] With such an arrangement, the voltage applied to the cathode
terminals of the light-emitting diodes 300R, 300G, and 300B is set
to as low a value as possible. Such an arrangement reduces the
output voltage Vout of the switching regulator 130, thereby
reducing power consumption of the constant-current circuits 210R,
210G, and 210B.
[0053] The driver circuit 120 generates a driving signal Vdrv that
alternately provides a high-level state and a low-level state,
which can be switched according to the PWM signal Vpwm. The driving
signal Vdrv is input to the gate terminal of the switching device
SW1, thereby controlling the switching operation.
[0054] The second feedback voltage Vfb2 input to the second
feedback input terminal 106 is input to the driver circuit 120. The
driver circuit 120 makes a comparison between the second feedback
voltage Vfb2 and a predetermined threshold voltage Vth. In a case
that the Vfb2 is greater than the Vth, the driving signal Vdrv is
set to a fixed low level state so as to stop the switching
operation of the switching element SW1, whereupon the switching
regulator 130 stops the step-up operation.
[0055] Next, description will be made regarding an arrangement in
which the light-emitting device 400 having the above-described
configuration is mounted to the cellular phone terminal 1000.
[0056] The circuit components enclosed by the broken line 1100 in
FIG. 2 are mounted within the first housing 1100 shown in FIG. 1.
In the same way, the circuit components enclosed by the broken line
1200 in FIG. 2 are mounted within the second housing 1200 shown in
FIG. 1. On the other hand, the portion enclosed by the broken line
1300 corresponds to the connection portion 1300 shown in FIG. 1,
which includes two connection wiring lines W1 and W2.
[0057] The liquid crystal panel 1400 of the cellular phone terminal
1000 is provided on the first housing 1100 side. Accordingly, the
light-emitting diodes 300R, 300G, and 300B, which are used as a
backlight for the liquid crystal panel 1400, are mounted on the
first housing 1100 side.
[0058] On the other hand, the battery 500 of the cellular phone
terminal 1000 is provided on the second housing 1200 side.
Accordingly, the switching regulator 130 for boosting the battery
voltage Vbat output from the battery 500 and the control circuit
100 for the switching regulator 130 are provided on the second
housing 1200 side. Such a configuration reduces the area necessary
for the wiring of the power supply lines.
[0059] Here, the inductor L1 and the output capacitor C1 included
in the switching regulator 130 are provided in the form of chip
components having a relatively large height or a relatively large
area. However, with such an arrangement, the second housing 1200
has a margin of available volume capacity. Accordingly, in many
cases, the size of the chip components does not lead to a problem
of difficulty in mounting the chip components within the second
housing 1200, as compared with mounting the chip components within
the first housing 1100.
[0060] With the light-emitting device 400 according to the present
embodiment, the current driving circuit 200 for controlling the
brightness of the light-emitting diodes 300R, 300G, and 300B is
provided on the first housing 1100 side. The cathode terminals of
the light-emitting diodes 300R, 300G, and 300B are connected to the
current driving circuit 200 via three wiring lines. With such an
arrangement, all of these wiring lines are provided within the
first housing 1100, thereby confining the wiring area to the
minimum necessary.
[0061] A first wiring line W1 that connects the switching regulator
130 and each of the light-emitting diodes 300R, 300G, and 300B is
provided between the first housing 1100 and the second housing 1200
via the connection portion 1300. The output voltage Vout of the
switching regulator 130 is supplied to the light-emitting diodes
300 via the first wiring line W1, thereby driving the
light-emitting diodes 300.
[0062] Also, a second wiring line W2 that connects the feedback
output terminal 208 of the current driving circuit 200 and the
first feedback input terminal 104 of the control circuit 100 is
provided between the first housing 1100 and the second housing 1200
via the connection portion 1300 in the same way as with the first
wiring line W1.
[0063] Furthermore, with the light-emitting device 400 according to
the present embodiment, the control circuit 100 generates a
switching signal while monitoring the output voltage Vout of the
switching regulator 130. In order to perform such an operation, a
third wiring line W3 is provided for connecting the output of the
switching regulator 130 to the second feedback input terminal 106
of the control circuit 100. With such an arrangement, the third
wiring line is provided within the second housing 1200.
[0064] With the light-emitting device 400 having such components
mounted as described above according to the present embodiment, the
control circuit 100 and the current driving circuit 200 are
provided in the form of separate integrated circuits. Such an
arrangement improves the degree of freedom in the layout, thereby
further providing the following advantages.
[0065] With the light-emitting device 400 according to the present
embodiment, the switching element SW1, the inductor L1, the
rectifier diode D1, and the inductor L1 of the switching regulator
130, which are provided outside the LSI in the form of external
components, are mounted on the second housing 1200 side. In many
cases, such external components have a height that is larger than
the thickness of the semiconductor integrated circuit. Thus, such
an arrangement provides the first housing 1100 with a reduced
thickness. On the other hand, before mounting the switching
regulator 130, the second housing 1200 includes components that
have a relatively large thickness, such as the battery 500 and so
forth. Accordingly, in many cases, mounting the switching element
SW1 and so forth on the second housing 1200 side does not require
an increase in the thickness of the second housing 1200.
[0066] Thus, such an arrangement provides the cellular phone
terminal 1000 that is slimmer and smaller overall.
[0067] Furthermore, the light-emitting device 400 according to the
present embodiment includes two wiring lines, i.e., the first
wiring line W1 and the second wiring line W2, provided to the
connection portion 1300 having a hinged structure.
[0068] On the other hand, let us consider an arrangement in which
the control circuit 100 and the current driving circuit 200 are
integrally provided in the form of a single integrated circuit.
With such an arrangement, in order to provide the first housing
1100 with a reduced size and a reduced thickness, all of the
control circuit 100, the current driving circuit 200, and the
switching regulator 130 should be mounted on the second housing
1200 side. However, with such an arrangement, there is a need to
provide three wiring lines that connects the cathode terminals of
the multiple light-emitting diodes 300R, 300G, and 300B, and the
terminals 230R, 230G, and 230B of the current driving circuit 200
across the connection portion 1300. Furthermore, there is a need to
provide the additional wiring line W1 that connects the switching
regulator 130 and the anode terminals of the light-emitting diodes
300R, 300G, and 300B. Accordingly, there is a need to provide at
least four wiring lines within the connection portion 1300.
[0069] The area necessary for the wiring increases according to the
increase in the number of wiring lines that connect the first
housing 1100 and the second housing 1200 via the connection portion
1300. Accordingly, the increased number of wiring lines is one
factor hindering the miniaturization of the cellular phone terminal
1000. This means that the number of wiring lines that pass through
the inside of the connection portion 1300 is preferably confined to
the minimum necessary. With the light-emitting device 400 according
to the present embodiment, the number of such wiring lines is
confined to two, which is the minimum necessary, and which
corresponds to the first wiring line W1 and the second wiring line
W2. Thus, such an arrangement confines the area of the housing
necessary for the wiring to the minimum necessary.
[0070] Furthermore, the wiring lines are provided within the
connection portion 1300 in the form of an FPC (Flexible Printed
Circuit), which has poor strength as compared with the wiring
provided on a PCB (Printed Circuit Board).
[0071] Accordingly, the number of wiring lines that pass through
the inside of the connection portion 1300 is preferably confined to
the minimum necessary. With the light-emitting device 400 according
to the present embodiment, the number of such wiring lines is
confined to two, which is the minimum necessary, and which
corresponds to the first wiring line W1 and the second wiring line
W2. Thus, such an arrangement also suppresses adverse effects on
reliability.
[0072] Also, with the light-emitting device 400 according to the
present embodiment, the third wiring line W3 is provided within the
second housing 1200 for inputting the output voltage Vout of the
switching regulator 130 to the second feedback input terminal 106
of the control circuit 100 as a feedback signal.
[0073] Now, let us consider a case in which the second wiring line
W2 provided within the connection portion 1300 has broken. In this
case, the control circuit 100 receives the first feedback voltage
Vfb1 of 0 V as a feedback signal. Accordingly, the control circuit
100 operates so as to control the ON/OFF state of the switching
element SW1 such that the first feedback voltage Vfb1 approaches
the predetermined reference voltage Vref. This can increase the
output voltage Vout beyond the voltage necessary for driving the
light-emitting diodes 300R, 300G, and 300B, leading to adverse
effects on the reliability of the overall device.
[0074] With the light-emitting device 400 according to the present
embodiment, the third wiring line W3 is only provided within the
second housing 1200, thereby suppressing the risk of breakage of
the third wiring line W3. With such an arrangement, the control
circuit 100 controls the switching operation of the switching
element SW1 such that the output voltage Vout input from the third
wiring line W3 as a feedback signal does not exceed the
predetermined threshold Vth. Such an arrangement prevents the
output voltage Vout from increasing excessively, even in a case
that the second wiring line W2 has broken, thereby improving the
reliability of the cellular phone terminal 1000.
[0075] Also, with the light-emitting device 400 according to the
present embodiment, the voltage of the cathodes of the
light-emitting diodes 300R, 300G, and 300B are used as a feedback
signal. Specifically, control is performed such that the cathode
voltage approaches the predetermined voltage Vref. With such an
arrangement, the reference voltage Vref is set to a voltage value
around the minimum necessary for operating the constant-current
circuits 210R, 210G, and 210B. Such an arrangement provides the
minimum necessary output voltage Vout generated by the switching
regulator 130, thereby providing the high-efficiency light-emitting
circuit 400.
[0076] The above-described embodiments have been described for
exemplary purposes only, and are by no means intended to be
interpreted restrictively. Rather, it can be readily conceived by
those skilled in this art that various modifications may be made by
making various combinations of the aforementioned components or
processes, which are also encompassed in the technical scope of the
present invention.
[0077] With the present embodiment, the control circuit 100 and the
switching element SW1 may be integrally provided in the form of a
single unit. Also, the control circuit 100 and other circuit
components may be integrally formed. The circuit components to be
integrally formed should be decided on the basis of costs, the
amount of space they occupy, etc.
[0078] Description has been made in the present embodiment
regarding the cellular phone terminal 1000 as an example of the
information terminal device. Also, the present invention can be
broadly applied to folding information terminal devices that
require a liquid crystal backlight, such as PDAs, second-generation
cordless telephone systems, etc.
[0079] While the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
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